Typically, printed circuit boards are prepared by stacking a plurality of prepregs in various arrangements, followed by pressing at high temperatures (e.g. greater than 170° C.). The prepregs consist of a partially cured curable resin coated onto fiber reinforcement, typically glass. A “partially cured” curable resin is known in the art as a “B-staged” resin. This partial curing or “B-staging,” raises the glass transition temperature (Tg) of the thermoset above ambient temperature (20° C. to 30° C.); whereby the Tg can be from 30° C. to 100° C. so that the prepregs can be rolled up without sticking. When the prepregs are stacked, pressed, and heated to achieve final cure the resin can flow to consolidate the layers before final cure (typically, final cure occurs when the Tg of the composition does not vary by more than 5° C. as measured by differential scanning calorimetry).
The ability to B-stage a resin is essential to the process of manufacturing printed circuit boards. A B-staged resin is a resin which has a portion of its curable moieties reacted for example, anywhere from 1 mol % to 95 mol %, of the resin's curable moieties; and wherein the “gel point” of the resin has not been reached. (The “gel point” is defined as the point at which a liquid formulation begins to exhibit elastic properties and increased viscosity). The “gel point” of a curable resin is the point along the cure profile at which an infinite network forms. Although further cure can occur, the resin will no longer flow. A B-staged resin can melt and flow during subsequent processing and further heating. During this latter process, called “C-staging” or “final cure”, the resulting thermoset is crosslinked beyond the gel point and will no longer flow. At this C-stage, for example, typically more than 90 mol % of the curable moieties of the resin have reacted.
Typically, a resin formulation used to make prepregs is dissolved in a solvent. This resulting solvent solution is called a “varnish” or “dope”. The solvent is required in a resin formulation because the solvent is used to reduce the viscosity of the resin formulation, for example, below 1 Pa·s such that good fiber wetting (impregnation of the fibers with the resin without the presence of “dry spots” on the fiber) is achieved. When the wetting of the fibers is adequate, the resultant prepregs made from the resin formulation are void-free and the surface of the prepregs is smooth.
After a fiber reinforcement is wetted with a varnish, the wetted fiber reinforcement is usually passed through a ventilated oven to both evaporate the solvent and partially cure (B-stage) the resin. With known processes, it is critical to control the extent of cure during B-staging. For example, if insufficient cure is achieved (e.g., <20 mol % of the resin's curable moieties are reacted), the resin will flow too much such that the resin does not impregnate the fiber successfully, and instead the resin flows straight through the fibers, leaving the fibers dry during final cure. For example, for the preparation of electrical laminates, the dryness of the fibers is such that the fibers have less than 40 wt % resin at the B-stage. Thus, a part to be pressed having a low resin/fiber ratio, causes the pressed parts to be too thin and valuable resin is lost. For example, in the preparation of electrical laminates, if more than 10% of the resin is forced beyond the edge of a fiber reinforcement such as a reinforcing fabric, the resin is considered to have too much flow.
On the other hand, if too much cure occurs during B-staging (i.e., the resin is at or past its gel point), the layers of parts to be pressed will not flow together during pressing, which (i) causes poor adhesion between plies (e.g., the plies easily come apart, and no flow of resin is apparent during the curing, and the laminate is not “transparent”); and (ii) causes voids to be formed in the cured product.
As aforementioned, a formulation used to prepare a prepreg material, includes the use of a solvent in the formulation. A significant part of the cost of preparation of prepregs is related to the cost of the solvent, the cost of the energy needed to evaporate the solvent from the prepregs, and the cost of the energy needed to incinerate the solvent vapors before the vapors are released into the environment. Therefore, a formulation and process for making prepregs that do not use a solvent (i.e., a solventless or solvent-free formulation and process) would be beneficial to the industry.
Solvent-free methods for making prepregs have heretofore been described, but none of the known solvent-free methods offer a combination of (i) the use of a low initial resin viscosity (e.g., <10,000 cP mPa-s), and (ii) the ability to B-stage prepregs. Therefore, there continues to be a need for B-staging curable resins for prepreg production in a multistage process without the use of solvent in the B-stageable formulation.
Current B-staging processes include promoting polymerization reaction of part of the resin starting material and suspending the polymerization reaction at an appropriate B-stage. For instance, a polymer composition which is formed in a first stage of curing by combining a base molecule and a linker molecule, results in a prepolymer which still has to be further cured in a second, third or further curing stage. A notable drawback to this process is lack of reproducibility to consistently reach the same B-stage or the same degree of polymerization, especially when a radical polymerization step is required in an overall curing process.
For example, in the aerospace and sporting goods industry, where prepregs are typically made with carbon fibers, a solventless process is used which involves using a hot-melt technique in order to impregnate the fibers with resin. The thickness of the prepreg is finely controlled by using calender rollers. No solvent is used in this process as structural composites made using these prepregs needs to be below a certain void content such as typically, <1%. The prepreg produced in this manner undergoes very little cure (e.g., <30% of the reactive moieties) during the prepregging process. The level of tack is controlled primarily by controlling the viscosity of the starting formulation at the prepregging temperature and the storage temperature. The main disadvantage of this type of prepreg is the need to transport the prepreg in refrigerated or cryogenic containers in order to prevent the tacky prepreg from undergoing further cure. Continued cure results in a loss of tackiness as well as causes issues during the subsequent laminate cure. In addition, once the prepreg needs to be used, the prepreg has to be warmed up back up to ambient temperature which adds additional work (and cost) to the process cycle. Most often these prepregs are stacked in specific stacking sequences depending on the design requirements for a specific use; and are cured in an autoclave under heat and pressure. It would therefore be ideal to provide a resin composition that can be used to form a tacky prepreg that is stable at ambient temperature. Typically, a storage stable resin composition includes a B-stageable material that will not continue to significantly crosslink during storage so as to facilitate shipping at ambient temperature.